Recently, a new type of pump capable of dispensing hand cleansers with mechanical scrubber in a foam format through a non-aerosol dispensing system has been developed (U.S. Pat. Nos. 8,002,151 and 8,281,958). This pump is an integral part of a platform that has allowed for the creation of a new hand cleanser category. This category is foam soap with mechanical, scrubbers.
Prior to the development of a pump that was capable of creating foam with mechanical scrubbers, existing foam pumps such as those described in U.S. Pat. Nos. 5,445,288 & 6,082,586 had the limitation of dispensing foam only. The reason for this is that standard foaming technologies create the foam by passing liquid and air through a porous media to generate the foam. If this technique was employed to create foam with mechanical scrubbers, the pump would simply ‘sieve’ the scrubbers from the liquid and cease to operate. A key characteristic of the hand cleansers dispensed from this type of pump is low viscosity. The viscosity of this form of hand cleanser is generally less than 100 cPoise and is tailored to be easily mixed with air through a porous media to produce foam from a pump.
The hand cleanser characteristics required to create foam with mechanical scrubbers are very different. If the hand cleanser is too thin (viscosity too low) and has a Newtonian rheological behaviour, the mechanical scrubbers will fall out of suspension. If the product is too thick (too viscous), the amount of force required to foam the formulation becomes too high resulting in excessive operating force for the dispenser user and a poor quality foam results. The viscosity range of this type of hand cleanser is generally between 500 cPoise and 4000 cPoise.
Typical non-aerosol foam pumps operate by pumping both air and liquid simultaneously. In essence the foam pump is a combination two pumps (an air pump and a liquid pump) working in tandem to bring a predetermined volume of air together with a predetermined volume of liquid. Since air is generally introduced into the liquid, the viscosity of the liquid will impact on the ability of the air to efficiently infuse. The resistance to infusion translates into back pressure being generated within the pump.
The efficiency of the infusion process is also limited by the simultaneous action of pumping the air into the liquid. Air is a compressible medium whist the liquid is not. Therefore when the air and liquid are being pumped the air compresses due to the resistance applied to it as it is being forced to infuse into the liquid. The result of this is variable foam quality where the ratio of air to liquid is lower at the start of the pumping process and higher at the end of the pumping process. For the pump user, this means the foam generated at the start of the pumping process is wetter than it is at the end. This condition is even more pronounced if a bellows pump or a diaphragm pump is used. These types of pumps deform as they collapse and during the deformation phase, little to no air is being delivered to a mixing chamber and thus the resultant foam is watery at the beginning part of the stroke. This problem is largely overcome with piston pumps for both the air and liquid. However, with a foaming element that includes a sparging element it would be advantageous to build up air pressure on the air side of and within the sparging element before liquid is delivered to the foaming element. Another issue that arises when attempting to foam higher viscosity foam soaps with mechanical scrubbers (as described above) using a foaming element that includes a sparging element is the ability to provide sufficient dwell time to maximize the air infusion process to create a high quality foam.